1. Field of the Invention
This invention relates generally to a method and structure for producing narrow openings to the surfaces of materials, and, more particularly, relates to a method and structure which is well suited for manufacturing integrated-circuit semiconductor devices having fine geometry patterns.
2. Description of the Prior Art
Techniques for producing narrow openings to the surface of a material are known. One of the most popular techniques, which is widely used in the semiconductor industry, involves forming a layer of photoresist on the surface of a material, selectively exposing portions of the photoresist to ultraviolet light and developing the exposed photoresist. The widths of openings produced using this technique are limited by diffraction and reflection effects at the wavelengths of the radiation used to expose the photoresist. Narrower openings can be produced by analogous methods employing different photoresist materials and radiation of shorter wavelength such as an electron beam or X-rays. A thorough discussion of the limitations of conventional photolithography can be found in the July 1975 issue of the IEEE Transactions on Electron Devices.
Another technique used to produce a narrow emitter opening in a semiconductor device is disclosed in a paper titled "A New Sub-Micron Emitter Formation with Reduced Base Resistance for Ultra High-Speed Devices" by H. Kamioka et al, presented in December 1974 to the International Electron Devices Meeting held in Washington, D.C. and published starting on page 279 in the technical digest of that meeting. Kamioka et al's technique forms a three-micron-wide layered sandwich of silicon nitride, silicon dioxide and silicon nitride centered over the desired location of the emitter opening on the surface of a silicon substrate. The sandwiched layer of silicon dioxide is then laterally etched inward from both sides to form with the two nitride layers a structure with "I-beam" cross-sectional configuration. The vertical rib of remaining silicon dioxide protectively masks an underlying ribbon (stated to be 0.5 micron wide) of silicon nitride while the exposed portions of silicon nitride are etched away from both sides. The overlying masking silicon dioxide rib is subsequently removed and a layer of silicon dioxide is formed on the exposed surface of the silicon substrate. The remaining ribbon of silicon nitride, which defines both the width and location of the narrow emitter opening, is then removed, thereby exposing a portion of the silicon substrate surface.
The selective lateral etching of a small-area-bounding lateral edge on an adjacent overlying layer of one material to expose a larger area of the underlying material is shown in U.S. Pat. No. 3,783,047 issued to M. M. Paffen et al on Jan. 1, 1974 and titled "Method of Manufacturing a Semiconductor Device and Semiconductor Device Manufactured by Using Such a Method." The method taught by Paffen et al is used to produce a semiconductor device having a small zone with one selected set of electrical properties and a larger zone with another set of electrical properties.
The use of a selective lateral etch is described by C. N. Berglund et al in a paper entitled "Undercut Isolation A Technique for Closely Spaced and Self-Aligned Metalization Patterns for MOS Integrated Circuits." This paper was published in September, 1973 beginning on page 1255 of Vol. 120, No. 9 of the Journal of the Electrochemical Society. C. N. Berglund et al take advantage of the shadowing effect of an undercut area etched in a two-layer-insulator sandwich. Because of the masking effect of an undercut edge a thin metal film evaporated at an appropriate angle to the edge will be discontinuous at the undercut edges, resulting in electrically isolated metalization patterns at different vertical levels with negligible lateral spacing between them. Berglund et al illustrate an application for this technique by describing the design of a two-phase CCD (Charge-Coupled Device).
Although electron-beam and X-ray lithography techniques can be used to produce narrow openings which make possible smaller semiconductor elements and higher chip density than previously obtained using less advanced methods, these techniques have many disadvantages. Not only is the equipment required to practice the electron-beam technique presently very expensive, but the technique also presently requires prolonged photoresist exposure times which are unsuitable for mass production. Optimized equipment required to practice X-ray lithography on a production scale is not currently available. Moreover, in addition to the well-known hazards and fail-safe precautionary measures associated with the use of X-rays, the technique requires a high precision mask often made of heavy metal, such as gold, possessing geometries as fine as those to be produced on the surface of the material. Such masks are generally produced with electron-beam techniques and are both expensive and difficult to produce.
The width of narrow openings producible with the doublesided etch method of Kamioka et al is limited by the fact that the silicon dioxide rib must be sufficiently wide to support the overhanging layer of silicon nitride. Additionally, although the lateral etch rate of silicon dioxide is in theory controllable to render the depth of undercut a function of etch time, as the depth of undercut is increased to produce an ever-narrower silicon dioxide rib, the difficulty in controlling etch rate, etch uniformity and hence the width of the to-be-formed opening increases. Further, as the etch process occurs simultaneously from two sides, the uncertainty in the width of the to-be-formed opening increases due to the combined uncertainty in the locations of the converging edges at any given time.